The present invention relates to an airbag gas generator to protect a passenger against impacts and more particularly to an airbag gas generator characterized by its unique operating performances.
Motor vehicles such as automobiles have an airbag system which, when a vehicle crashes at high speed, rapidly inflates an airbag with a gas to prevent a passenger from hitting against hard or dangerous portions inside the vehicle, such as steering wheel and windshield, due to inertia and thereby protect the passenger from getting injured or killed.
Such an airbag system should preferably be able to safely hold the passenger, whatever his or her build (which may vary depending on the passenger""s sitting height or whether the passenger is an adult or child) or the riding posture (some drivers may cling to a wheel). To meet this requirement, airbag systems have conventionally been proposed which operate to apply as small an impact as possible to the passenger at the initial stage of activation. JP-A 8-207696 proposes an airbag system that uses two kinds of gas generating capsules to produce a gas in two stages, with the first stage inflating the bag relatively slowly and the second stage causing a rapid gas generation. This system, however, has a drawback that the structure inside the gas generator is complicated, which in turn increases the size of a container and therefore the manufacturing cost.
U.S. Pat. No. 4,998,751 and U.S. Pat. No. 4,950,458 also propose airbag systems that have two combustion chambers to burn a gas generating agent in two stages to restrict the operating function of the gas generator. These systems, however, are complex in structure and not satisfactory.
The present invention provides an airbag gas generator which, though simple in construction, operates to apply as small an impact as possible to a passenger in the initial stage of operation and, in a subsequent operation stage, can reliably protect the passenger. For the driver""s seat, for example, this invention provides an airbag gas generator that moderates the inflation speed of the airbag during a period of 10 milliseconds from the start of the inflator activation compared with that of the conventional inflator and which, 30-50 milliseconds after the activation, exhibits an operation performance capable of holding the passenger well.
The above objective of this invention can be achieved by an airbag gas generator which comprises: a single ignition means activated by impacts; and a gas generating agent ignited and burned by the ignition means to generate a combustion gas, the ignition means and the gas generating agent being accommodated in a housing having a gas discharge ports; wherein an operation performance of the gas generator is adjusted such that the tank pressure measured at 0.25xc3x97T milliseconds will be 0.25xc3x97P (kPa) or less, where a desired tank maximum pressure in the tank combustion test is P (kPa) and a period of time from the start of rising of the tank pressure to the time when the f-maximum pressure P (kPa) is reached is T milliseconds. The operation performance described above is further preferably adjusted so that the tank pressure measured at 0.80xc3x97T milliseconds will be 0.70xc3x97P (kPa) or higher.
In particular, this invention provides an airbag gas generator in which a peak of the combustion maximum internal pressure during the gas generator operation occurs 10-20 milliseconds after an ignition current is supplied.
The peak of the combustion maximum internal pressure in this gas generator during operation should preferably occur 12-16 milliseconds, or more preferably 13-15 milliseconds, after the ignition current is applied. The gas generator of this invention gradually discharges the combustion gas of the gas generating agent from gas discharge ports to moderately increase the pressure of the gas generator and the pressure in the tank until 10-20 milliseconds after the application of the ignition current, at which time the combustion maximum internal pressure peaks. After the pressure in the housing of the gas generator has peaked, the gas in an amount sufficient to hold the passenger is rapidly discharged from the gas discharge ports to lower the internal pressure of the gas generator and at the same time sharply increase the pressure in the tank. As a result, an airbag gas generator is realized, whose operation performance is adjusted such that the tank pressure measured at 0.25xc3x97T milliseconds is 0.25xc3x97P (kPa) or lower as described above, where a desired tank maximum pressure in the tank combustion test is P (kPa) and a period of time from the start of rising of the tank pressure to the time when the maximum pressure P (kPa) is reached is T milliseconds. With the gas generator of this invention with the above operation performance, the output during the initial stage of operation is limited, so that the airbag (bag body) accommodated in the module can be prevented from rapidly inflating in the initial stage of operation and applying excess impacts to the passenger. On the other hand, if the tank pressure measured at 0.25xc3x97T milliseconds is 0.25xc3x97P (kPa) or higher, the force with which the bag inflates to break open the module is too strong, making it difficult to produce a desired effect of this invention.
The present invention in particular provides an airbag gas generator comprising: a single ignition means activated by impacts; and a gas generating agent ignited and burned by the ignition means to produce a combustion gas, the ignition means and the gas generating agent being accommodated in a housing having gas discharge ports; wherein an operation performance of the gas generator is adjusted such that the tank pressure measured at 0.25xc3x97T milliseconds is 0.25xc3x97P (kPa) or lower, or more preferably the tank pressure measured at 0.80xc3x97T milliseconds is 0.70xc3x97P (kPa) or higher, where a desired tank maximum pressure in the tank combustion test is P (kPa) and a period of time from the start of rising of the tank pressure to the time when the maximum pressure P (kPa) is reached is T milliseconds; and wherein the peak of the combustion maximum internal pressure during operation occurs 10-20 milliseconds or preferably 12-16 milliseconds or more preferably 13-15 milliseconds after the application of ignition current.
The tank combustion test in this invention refers to a test performed in a method described in the following.
 less than Tank Combustion Test greater than 
An airbag gas generator is fixed in a SUS (stainless steel) tank with an internal volume of 60 liters. After the tank is hermetically closed at a room temperature, the gas generator is connected to an external ignition electric circuit. A pressure transducer installed in the tank is used to measure a change in the increasing pressure for a duration from 0 to 200 milliseconds, with the moment at which the ignition electric circuit switch is turned on (the ignition current is applied) taken as time 0. The measured data is then processed by a computer to generate a tank pressure/time curve (hereinafter referred to as a xe2x80x9ctank curvexe2x80x9d) which is used to evaluate the performance of the gas generating pellets. After the combustion is over, a part of the gas in the tank may be sampled for CO and NOx analysis.
The tank maximum pressure in this invention refers to a maximum pressure in the stainless steel tank during the tank combustion test, and the combustion maximum internal pressure refers to a maximum pressure in the housing when the gas generator is operated.
The airbag gas generator of this invention has a simple construction and operates to apply as small an impact as possible to the passenger during the initial stage of its operation and thereafter rapidly inflate the airbag to reliably protect the passenger.